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Barometer - Mercury Barometers, Aneroid Barometer, The Altimeter

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A barometer is an instrument for measuring atmospheric pressure. Two kinds of barometers are in common use, a mercury barometer and an aneroid barometer. The first makes use of a long narrow glass tube filled with mercury supported in a container of mercury, and the second makes use of a diaphragm whose size changes as a result of air pressure.

Modifications to the mercury barometer

The barometer described above is adequate for making rough measurements of atmospheric pressure. When more accurate readings are needed, however, modifications in the basic design of the barometer must be made. The most important factor to be considered in making such modifications is changes that take place in the mercury reservoir at the bottom of the barometer as a result of changes in atmospheric pressure.

When the atmospheric pressure decreases, for example, air pressure is able to sustain a slightly smaller column of mercury, and some mercury flows out of the glass tube into the reservoir. One might hope to find the An antique aneroid barometer. Photograph by Diana Calder. The Stock Market. Reproduced by permission.

new pressure by reading the new level of the mercury in the glass tube. However, the level of the mercury in the glass tube must be compared to the level of the mercury in the reservoir, and the latter has changed also as a result of a new atmospheric pressure.

This problem is dealt with in one of two ways. In one instrument, the English Kew barometer, no modification is made in the mercury reservoir itself. Instead, changes that take place in the mercury level in the reservoir as a result of changes in atmospheric pressures are compensated for by making small changes in the measuring scale mounted to the glass tube. As one moves upward along the scale, the graduations between markings become slightly smaller to correct for the changing level of the mercury in the reservoir.

A second type of barometer, the Fortin barometer, contains a flexible bag that holds an extra supply of mercury metal. The flow of mercury into and out of that bag and then out of and into the glass tube is controlled by an adjustable screw whose point is moved so as just to touch the surface of the mercury in the reservoir. As atmospheric pressure and mercury levels change, modifications of the adjustable screw keep the mercury level at a constant height.

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